When I open Software Update, I have:
An update is available for your Mac
- macOS Catalina 10.15.4 Update
When I click on "Update Now", I have a window with:
To update, you must restart your Mac.
If I click restart, I effectively restart but when I try to update it asks to restart again.
I have a MacBook Pro (Retina, 13-inch, Early 2015) and 12.29GB available.